Use of Morphology and Endocrinology to Predict Sex in California Sheephead: Evidence of Altered Timing of Sex Change at Santa Catalina Island, California

Abstract

To estimate the health of the California sheephead Semicossyphus pulcher fishery, a 2004 stock assessment used available biological data that were collected decades prior to an increase in fishing pressure. However, a recent study has found that sex ratios, growth rates, survivorship, and average sizes of females and males have changed in response to size-selective fishing in some California sheephead populations. To better understand the potential changes in protogynous California sheephead, this study sought to determine (1) whether external morphology was still an accurate method of predicting sex in sexually dimorphic California sheephead at Santa Catalina Island, California, and (2) whether nonlethal blood sampling and plasma hormone analysis could be used to predict sex for future stock assessments. Sex was determined using gonadal histology and compared with several specific external morphological characters. Estradiol and 11-ketotestosterone plasma concentrations were also compared across sexed individuals. The rate of error when using external morphology alone to predict sex was 58%. In contrast, sex steroid concentrations varied significantly across sexes; estradiol concentrations were significantly greater in females than in transitioning and male individuals during the breeding season, and 11-ketotestosterone concentrations were significantly lower among females. Gonadal histology showed that 21% of the fish caught during the breeding season were classified as transitional, in stark contrast to historical data. The inability to accurately predict sex using external morphology alone suggests that commonly used methods of surveying California sheephead populations (e.g., diver surveys) may be inaccurate. Nonlethal blood sampling and subsequent plasma hormone analysis may offer an alternative method for assessing sex in California sheephead. Because California sheephead are not reproductively functional during transition and because we found such a large proportion in transition during the breeding season at Santa Catalina Island, we believe there is a need for continued assessment of the reproductive potential in this population of California sheephead.